Kal Penn, who plays Kumar, is allergic to nuts. During the scene where he crashes through the ceiling at the station, a nut-based mixture was used for the dust that comes out of the heat vent. Upon entering the set that day, Penn had a severe allergic reaction, and had to be taken to a hospital. Later, a safer, nut-free concoction was used for the scene.

The director of 'Harold and Kumar' also directed 'Dude, Where's My Car?'. This is given a nod near the end of the film where the guys meet up with Neil Patrick Harris again, Harold says "Dude where's my car?"

It was mentioned a few times during the movie that "Sixteen Candles" (1982) is one of Harold's favourite movies, but in real life, John Cho (Harold) really hates that movie. He was traumatised when he first saw it because of the humiliating portrayal of the Asian character (Long Duck Dong).

Kal Penn, who plays Kumar, is a vegetarian, so special veggie Castles had to be made for him.

The two "sidekicks" of the movie, Goldstein and Rosenberg, have suspiciously similar names to the "sidekicks" of Hamlet, Rosencrantz and Guildenstern. Don't know if this is on purpose or not but it always seemed that way to me.

When Kumar (Kal Penn) is in the ER to save the gunshot victim, the nurse who wipes his brow is played by Ryan Reynolds, who starred with Penn and even had a similar scene with reversed roles in "Van Wilder".

When Kal Penn, who plays Kumar, went to the audition, the first thing they said to him was "Where's your turban?"

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When Harold and Kumar stop to take a pee and then leave, Harold gets bitten by a raccoon. In one shot Harold's blood stain is seen on his jacket, but in the next shot it's gone, then back again and so forth, until the car comes to a stop.